In many fields of use, there is a need to gauge surfaces of objects and hence also the objects themselves with high accuracy. This applies in particular to the manufacturing industry, for which the gauging and checking of surfaces of workpieces is very important.
For these applications, there is a number of existing measuring devices which are designed for specific tasks and are also designated as coordinate measuring devices or machines. These measuring devices gauge the surface by producing mechanical contact and probing the surface. Examples of this are gantry measuring machines, as described, for example, in DE 43 25 337 or DE 43 25 347. Another system is based on the use of an articulated arm whose measuring sensor arranged at the end of the multipart arm can be moved along the surface. Articulated arms of the generic type are described, for example, in U.S. Pat. No. 5,402,582 or EP 1 474 650.
In the prior art, a tactile probe which consists of a ruby sphere which is mounted on a measuring staff whose deflection is determined during probing via a switching element or distance-measuring element is used as a standard measuring sensor in such coordinate measuring devices. On the basis of the switching point or deflection distance, the point of contact is calculated. Although this approach is a mature solution for point measurements, this technical solution is suitable only for relatively slow measuring rates owing to the resultant frictional forces, probing forces and mechanical inertias. Moreover, impurities and abraded material on the sphere of the sensor and the object being measured lead to measuring errors. Precisely, for example, the internal measurement of drilled holes, however, requires precise and fast measurement in order to determine the waviness and roundness of the parts, particularly if these are to be carried out directly in the production line.
Approaches for non-contact gauging have therefore already been pursued in the prior art. The optical technologies used for this purpose in coordinate measuring machines are based firstly on cameras which determine dimensions of parts in reflected light or transmitted light, for example by means of the casting of shadows, with the aid of image recognition. Inner bores are however not characterizable in this manner; likewise, irregularities of surfaces are detectable only with the use of structured illumination.
Although triangulation-based systems permit very accurate distance determinations, the dimensions of the measuring head in the case of accuracies in the μm range and measured distances in the cm range are very large owing to the required base between the optical axes of transmitter and receiver. This also applies to confocal and chromatical confocal measuring principles which necessitate large dimensions of optical systems if a measuring range of a few cm is to be realized. Also problematic both in the case of triangulation and in the case of the confocal method is partial obscuration of the measuring or observation beam, which leads to large errors of measurement.
A further approach utilizes white light interferometry for high-precision gauging. Here, the application either employs scanning, i.e. by displacement of the interferometer, and therefore takes place slowly or, in the case of spectrally resolved detection, as a rule with limitation to a measuring range of a few mm.
A similar interferometric approach is disclosed, for example, in WO 92/19930, although in this case the recording of depth profiles of human tissue in the area of medical technology is of primary importance.
EP 1 744 119 discloses a system for gauging surfaces using optical coherence tomography and a frequency-modulated source. Here, a fibre ring laser is made tuneable by an acoustically tuneable filter element. The laser radiation is then used for interferometric gauging of surfaces in a common path interferometer, i.e. an interferometer which uses at least partly the same components or beam paths for measuring radiation and reference radiation. The reference distance here is provided by a reflection in the measuring arm of the interferometer. A reference interferometer is used for calibrating the wavelength. Although this setup is suitable in principle for fast, non-contact gauging of surfaces, it implicitly requires favourable geometrical conditions, as are present, for example, in the described gauging of cylindrical openings. This approach in this configuration is not suitable for the high-precision gauging of surfaces having arbitrary geometries and surface variations.